Passenger comfort and fuel efficiency have set forth increasing demands on automotive vehicle designs. It is a primary goal of most vehicle designs to provide a more efficient vehicle without having to sacrifice passenger comfort and satisfaction.
Moreover, and as alternative vehicle propulsion systems are implemented, passenger comfort and fuel efficiency are sometimes in opposition to each other. This is particularly true in hybrid vehicle designs.
A Hybrid Vehicle is a vehicle that has two sources of propulsion. A hybrid electric vehicle (HEV) is a vehicle wherein one of the sources of propulsion is electric and the other source of propulsion may be derived from fuel cells or an internal combustion engine (ICE) that burns diesel, gasoline or any other source of fuel.
Generally, a hybrid vehicle utilizes either one or two drive trains wherein the internal combustion engine (ICE) provides torque to one of the drive trains and an electrical driving force is applied to either of both of the drive trains.
The torque characteristics produced by an internal combustion engine are significantly different than those produced by an electric motor. For example, an electric motor provides higher torques at lower rpms while an internal combustion engine develops lower torques at lower rpms.
Generally, a conventional automatic transmission, clutch-to-clutch auto transmission and or manual transmission is coupled to an internal combustion engine. The transmission is positioned in the drive train between the internal combustion engine and the driven wheels. The transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
However, the drivability of a hybrid vehicle is adversely affected due to the torque oscillations that occur when abrupt torque changes are encountered in the operation of the internal combustion engine and the transmission coupled to it. Such oscillations are encountered during shifting, launching end starting and stopping of the engine in order to conserve fuel.
Accordingly, and in order to meet the torque demand of the automobiles acceleration, a transmission having multiple gear ratios must be coupled to an internal combustion engine.
Additionally, and as the transmission of an internal combustion engine shifts through its gear cycle, the engine driveshaft is generally disengaged from the transmission through a clutch mechanism which allows for the shifting of the gears. Once the gear transfer is complete the driveshaft is reengaged to the transmission.
Thus, the opening and closing of a clutch mechanism causes the drive train of an internal combustion engine to have a series of torque transfers with a steep drop-off or gap in between each series of transfer.
This presents a particular problem in hybrid vehicles where both an internal combustion engine and an electrically driven engine provide a driving force to the vehicle. Moreover, and in order to improve fuel economy, the internal combustion engine is frequently shut off and restarted. This cycling on off of the engine will also create a series of torque transfers with a speed drop-off or gap.
In order to provide a highly efficient hybrid vehicle that utilizes a fuel efficient internal combustion engine, the torque oscillations caused by a direct coupled drive train must minimized.
In addition, hybrid vehicles also utilize a concept known as regenerative braking. Generally, regenerative braking is the conversion of the vehicle's kinetic energy into a source of electrical power. The vehicle's kinetic energy is converted from the moving vehicle, in response to a user request to slow or stop the vehicle. A generator is manipulated, and accordingly, produces electrical energy as it applies a stopping force to the vehicle's axle and/or drive train in response to a stopping request.
Therefore, and in accordance with regenerative braking, the kinetic energy is converted to electric energy, as the vehicle begins to slow down.